[A11y] Create a11y page with placeholders. (#9889)

The screen is gated by the FeatureFlags.accessibility flag.

internal link: go/devtool-a11y-userjourney 

### General checklist

### Issues checklist

### Tests checklist

### AI-tooling checklist

    * [ ] I read the [AI contributions guidelines] and agree to follow them.
    * [ ] I reviewed all AI-generated code before opening this PR.
    * [ ] I understand and am able to discuss the code in this PR.
    * [ ] I have verifed the accuracy of any AI-generated text included in the PR description.
    * [ ] I commit to verifying the accuracy of any AI-generated code or text that I upload in response to review comments.

### Feature-change checklist 

    * [ ] I added the `release-notes-not-required` label or left a comment requesting the label be added.
    * [ ] I added an entry to `packages/devtools_app/release_notes/NEXT_RELEASE_NOTES.md`.
    * [ ] I included before/after screenshots and/or a GIF demo of the new UI to my PR description.
    * [ ] I ran the DevTools app locally to manually verify my changes.

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